1. Field of the Invention
This invention relates to a method and electrical circuitry for the reproduction of optical sound tracks recorded on photographic film, and more particularly to the reproduction of sound tracks recorded as variable density recordings for motion picture sound.
2. Description of the Prior Art
The sound for motion pictures is usually recorded photographically on the print film for the finished motion picture. To print this photographic sound track on the "release print", a photographic negative sound track is first recorded. The negative is then used to print the sound track onto the release print as the picture is being printed.
There are two types of photographic sound tracks in general use: variable area, and variable density. A "variable area" sound track print is recorded as a varying width clear area in the sound track portion of a film, as illustrated in FIG. 1. In this figure a piece of a motion picture print 2 is shown with a sound track area 4 upon which a varying width clear area 6 has been recorded. The area 8 within the sound track on either side of the clear area is opaque. The sound track is scanned by a slit 10 located on the opposite side of the film from a light source. The slit passes on light which has been transmitted through a narrow area of the sound track oriented transverse to the direction of film travel 12. Thus, as the film moves the amount of light passing through the film slit varies in response to the recorded sound modulation. This light is detected by a photoelectric cell whose output is processed and used to reproduce the recorded sound. This type of system is disclosed in my U.S. Pat. No. 4,328,574, issued May 4, 1982, which also discloses a detection system for crossmodulation distortion on the sound track.
A "variable density" sound track is illustrated in FIG. 2. The sound track area 14 is again scanned by a reading slit 16 as the film travels in direction 18. In this case the density or transmission of the track area is modulated uniformly across the entire width of the sound track. Variable density sound tracks are printed in a manner similar to that used to print variable area sound tracks. A negative is made with a light transmission capability that varies in response to the recorded sound, and used to expose the sound track portion of the print as the picture is being exposed onto the release print.
The ability of a piece of photographic film to transmit light is determined by the amount of light to which the film is exposed prior to development. Within a given range of exposure the transmission of a piece of film is related to its exposure by the relationship T=KE.sup.-g where
T=transmission of the film, PA0 K=a constant, PA0 E=exposure of the film in units of energy per area, and PA0 g=gamma, a constant relating the film's transmission to its exposure.
If a negative is exposed to an amount of light which varies directly with the amplitude of the sound to be recorded, the transmission of light through the negative is not linearly related to the original sound as represented by E, but rather is related by an exponential function. If the transmission of a negative thus recorded is read by a playback slit such as slit 16 in FIG. 2, the result will be a highly distorted signal. This distortion is ordinarily corrected by making a print whose gamma is such that it cancels the distortion.
Since light is transmitted through the negative to make the print and again through the print to reproduce the original sound signal, two transmission factors of KE.sup.-g are encountered. In the prior art an attempt has been made to select the negative and print films such that the product of their gammas is unity, thereby cancelling the non-linear transmission characteristics of the negative and print so that the finished print produces a faithful, distortionless reproduction of the original negative exposure. This process requires the making of a print whose gamma is exactly correct in order to faithfully reproduce a distortionless sound track. This is particularly difficult with many older negatives, which have unknown gammas, and which require prints to be made with characteristics that are difficult to obtain with contemporary film stocks.